Cohesive low density pellets and process of controlled drying



Patented Dec. 1, 1936 UNITED STATES 2,062,636 PATENT OFFICE COHESIVE LOW DENSITY PELLE'I'S AND PROCESS OF CONTROLLED DRYING pany, Wilmington, Delaware Del., a corporation of No Drawing. Application April 11, 1934, Serial No. 720,044

4 Claims.

This invention relates to a process for producing firm cohesive blocks or pellets of black blasting powder having improved physical and explosive properties, and more particularly to a process for producing pellets of unusually low density. The invention also relates to the new products of the process.

Black blasting powder, compressed into the form of cylindrical blocks or pellets, has been in troduced into practice in the past few years with considerable success, this form of powder largely replacing the glazed granular black blasting powder previously used. Such pressed powder commonly comprises pellets about 2 in length and of diameters varying from 1" to 2 Four of the above 2" pellets are ordinarily enclosed, end to end, in a paper wrapper. This package resembles a stick of dynamite and may be readily loaded into a bore hole.

The pellets just described are produced from ordinary black powder composition, for example 72% sodium nitrate, 12% sulfur, and 16% charcoal, by thoroughly incorporating the ingredients in the customary edge-runner wheel mills. The product of such an incorporation stage is commonly known as wheel cake. The wheel cake is pressed into slabs of high density, approximately 2 feet square and 1 inch thick, commonly known to the industry as press cake. Press cake is then corned or grained to a granulation suitable for pressing into pellets. A typically suitable granulation would be one passing through a screen having an opening of 0.115" and holding on a screen having an opening of 0.0406". The pellets are produced by feeding a granulation of the foregoing nature into a mechanical press where the material is given the desired cylindrical shape, a hole of diameter being provided during pressing, which runs longitudinally through the pellet.

The prior art methods of producing black blasting powder pellets have been concerned with the manufacture of a relatively high density product by the gravity feeding of granulated powder to the mechanical pelleting press. A suitable granulated black blasting powder has a specific gravity of about 1.76, and from such powder, pellet densities in the range of 1.55 to 1.65 are commonly obtained. However, the production of pellets of a density lower than 1.50 has, within recent times, become practical through the addition of special low density ingredients, for ex ample carbonaceous absorbents, to the usual constituents of black powder. Such a modified black blasting powder composition may be compressed and grained to specific gravities lower than 1.76, which materially increases their bulk and makes it possible to produce pellets of a density as low as 1.40.

While 1.40 is a practical low limit of density for pellets pressed from grained powder, lower other than pellet powder.

density may be obtained by pressing powder in other forms which have densities considerably lower than grained powder. would be the relatively low density mill or wheel cake, which has a heterogeneous structure, partly grainy but of composite density, in-the neighborhood of 1.5. The pelleting of such a material as wheel cake, with the use of suitable pressures, would allow the obtaining of pellets of lower densities than are commonly obtained from grained powder, but such pellets would be too fragile for handling in the manufacturing operations.

Wheel cake is already utilized in another possible method of producing a stick of black powder This method consists in punching moist wheel cake into a prepared cylindrical cartridge and subsequently drying the powder in the cartridge previous to closing the ends. While a firm package of black blasting powder similar in appearance to a stick of dynamite may be obtained by the above procedure, such a package has the disadvantage that the use of a fraction of a stick is inconvenient. Such powder is in the form of a continuous cylinder, whereas, with pelleted powder, one, two, or three pellets may be used as desired, with consequent economy in powder and proper loading of the bore hole. Cartridged wheel cake powder has the singular advantage of being a solidly formed cartridge of unusually low density.

The object of our invention is an improved process for the production of cohesive pellets of black blasting powder characterized by a relatively low density. A further object is such a process in which a firm surface is imparted to the pellets, thereby rendering them less fragile and capable of withstanding the necessary handling during manufacture. Additional objects will be disclosed hereinafter.

We have found that the foregoing objects are accomplished if a charge of black blasting powder of suitable moisture content is compressed into the predetermined form, for example, into a pellet 2" in length and of the desired diameter, and this pellet is then dried. While powder of varying initial moisture contents may be used, we have found that the most desirable content lies between 5 and 8%. Preferably, we take powder containing approximately a 6.5 moisture content, form it into the desired shape, and dry the pressed pellet to a final moisture content below 1%, preferably less than 0.5%. Using such a procedure, firm cohesive pellets are obtained, the surfaces of which are set sufficiently hard so that the pellets can be handled without chipping or breakage.

While various forms and compositions of black powder may be used in the foregoing procedure, the process is particularly adapted to the manufacture of pellets from black powder wheel cake,

Such a material since this form of material is especially suited to the production of low density pellets. In spite of the low density of wheel cake pellets, the fine bulky nature of the powder causes it to be relatively impervious.

While the drying procedure described in the foregoing gives a satisfactorily firm product, we find it advantageous at times to employ a preliminary drying process, in order that a hard surface may be imparted to the pellets at once, thereby making possible a saving in the immediate drying space necessary. If, for example, the pellets after pressing are permitted to remain on the press tray for a preliminary ten-minute drying period at 200 F., they are found to be firm enough on the surface for safe transfer to the final wire-bottomed or other forms of drying trays, where they are dried to a moisture content of less than 1%, and preferably to less than 0.5%.

We have also found that additional cohesiveness is given to the pellets if a small amount of binding material is incorporated with the other ingredients of the powder, previous to the pressing operation. The most desirable amount of such a binding agent for use has been found to be 0.2 to 2.0%, and preferably we employ starch for this purpose. A satisfactory composition of the dried pellet powder is the following:

Per cent Sodium nitrate 71.5 Sulfur 11.6 Charcoal 15.4 Chalk 0.5 Starch 1.0

The binding material which we prefer to incorporate is nonfibrous in character. In the place of starch, we may use various other suitable materials such, for example, as gum arabic, dextrin, and various agglutinants and gums, or even a combination of two or more of these materials.

In preparing pelleted powder of the foregoing description, we have found it desirable to feed to the pellet press wheel cake powder having a moisture between 5 and 8%, and preferably of approximately 6.5%. Such a powder is extremely difficult to feed by the usual methods, since it possesses no free flowing properties. We find it desirable, therefore, to make use of the feeding method described in the co-pending application of F. F. Chapman and C. R. Johnson, Serial No. 720,045, filed April 11, 1934, according to which this is accomplished by a process comprising the controlled movement of the wheel cake by means of high frequency mechanical vibrations.

In practice, therefore, our preferred procedure would be first to size the wheel cake properly and eliminate all lumps by passing it through a 6- mesh screen. The screened material, containing approximately 6.5% moisture, would then be fed to the pellet press by means of an apparatus imparting to the feed hopper a high frequency vibration movement, whereby a steady stream would be delivered in a controlled amount to the proper receptacle. The powder would then be compressed in the pellet press, carefully transferred to the preliminary drying trays, and subsequently be dried to a moisture content of preferably less than 0.5%. The finished pellets would subsequently be cartridged by enclosing two or more such pellets in carbonaceous or combustible material such as a paper wrapper. Pellets prepared in accordance with the foregoing process would be characterized to a marked degree by low density, namely from 1.0 to 1.4. Heretofore it has been impossible to obtain this result without the use of special low density ingredients.

The advantages of low density pellets according to our invention have already been mentioned. As compared with low density sticks of black blasting powder prepared by punching moist powder into preformed cartridges of approximately 8" in length, our pellets have the advantage that a portion of the stick can be readily used, when the powder can be removed from the wrapper, if so desired. The pellets produced according to our process will in general have a length not more than twice the thickness or diameter.

Our improved process and product have been described in detail in the foregoing. It should be understood, however, that many variations in procedure and in amounts of ingredients may be adopted without departure from the scope of our invention. We intend therefore to be limited only as indicated by the following patent claims.

We claim:

1. The process of producing a firm, low density, cohesive pellet of black blasting powder, which process comprises compressing into a container of predetermined form a charge of black powder wheel cake having a moisture content between 5 per cent and 8 per cent to produce a pellet sufficiently cohesive to withstand removal from said container, removing said compacted, undried pellet from said container, and drying said pellet to a moisture content less than 1 per cent, to produce a pellet which is non-friable in the unenclosed state.

2. The process of producing a firm, low density, cohesive pellet of black blasting powder, which comprises compressing into a container of predetermined form a charge of black powder wheel cake having a moisture content between 5 per cent and 8 per cent, to form a pellet having a compacted strength sufficient to enable it to withstand removal from said container in the undried state, removing said compacted, undried pellet from said container, preliminarily drying said pellet at elevated temperatures to impart a hard surface thereto, and subsequently drying said pellet to a moisture content of less than 0.5 per cent to produce a pellet which is non-friable in the unenclosed state.

3. The process of producing a firm, low density, cohesive pellet of black blasting powder, which comprises incorporating a non-fibrous binding agent with black powder wheel cake, compressing into a predetermined form a charge of said powder having a moisture content of approximately 6.5 per cent to form a pellet having a compacted strength sufiicient to enable it to withstand removal from said container in the undried state, removing said compacted, undried pellet from said container, drying said compacted pellet to a moisture content lower than 0.5 per cent to produce a pellet which is non-friable in the unenclosed state, and. subsequently encasing said pellet in a paper wrapper.

4. The process of claim 3, in which said nonfibrous binding agent consists of starch, present in an amount between 0.2 per cent and 2.0 per cent by weight.

CLARENCE W. BROOKS, JR. CHARLES R. JOHNSON. 

